1. Field of the Invention
The present invention relates to a photographic shutter for use in a single lens reflex camera wherein a motor is used to drive shutter blades in an exposure mode of operation.
2. Description of the Related Art
Upon depression of a shutter release button, a stepper motor is rotated in a clockwise direction to incrementally drive shutter blades from a closed orientation to an open orientation. When the level of a shutter aperture or opening has reached a value corresponding to the intensity of the scene brightness and the like, the stepper motor is rotated in a reverse or counterclockwise direction so as to close the shutter by the shutter blades. Such a shutter has been used as a lens shutter in a compact camera wherein a viewfinder is used to compose the content of a picture, said viewfinder having a path of light similar to that of an objective lens.
Unlike a focal plane shutter, such a lens shutter may conveniently be operated in synchronized relation to an electronic strobe in a flash exposure mode of operation. If the lens shutter is used in a signle lens reflex camera, then shutter blades must be kept open when a photographer composes the content of a picture through an objective lens. When the shutter release button is thereafter depressed, the shutter blades will be returned to a closed orientation. The stepper motor is then rotated to open and close the shutter blades in response to the intensity of the scene brightness and the like to complete an exposure mode of operation. If the shutter blades are turned to a closed orientation once the stepper motor, then the initiation of an actual exposure mode of operation is delayed as the limit of speed of the stepper motor M depends upon the nature of the stepper motor at high speeds. As a result, the content of an actual picture may differ from that of a picture that a photographer originally intends to compose.
In order to overcome the foregoing problems, the applicant of the present application previously proposed an improved shutter for use in a single lens reflex camera as shown in FIGS. 5 and 6 (see Japanese laid-open patent publication No. Sho 64/44924).
Specifically, the shutter includes a plurality of shutter blades (only one blade is shown in FIG. 5) designated at 101 and operable to open and close a shutter opening 102. A control member 103 is connected to the shutter blade 101 by a pin. Attached to one side of the control member 103 is a spring 104 by which the control member 103 is rotated in a clockwise direction. The control member 103 has another pin 105 at its other side.
A drive member 106 is operatively engaged with a stepper motor 108 and has a cam 107 which is, in turn, engageable with the pin 105. An engaging member 109 is rotatable around the axis of the drive member 106 and is engageable with the control member 103.
When the focus of an objective lens has been adjusted through the shutter opening 102, a shutter release button is depressed. The stepper motor 108 is correspondingly rotated in a counterclockwise direction between time t.sub.0 and time t.sub.1 as shown in FIG. 6. This causes the drive member 106 to move to the right as viewed in FIG. 5. Upon such movement of the drive member 106, the engaging member 109 is rotated to disengage from the control member 103. As a result, the control member 103 is rotated under the action of the spring 104 to allow the shutter blade 101 to rapidly close the shutter opening 102.
The stepper motor 108 is rotated in a clockwise direction between time t.sub.1 to t.sub.4 so as to move the drive member 106 to the left as viewed in FIG. 5. The pin 105 is then moved along the cam 107 to cause the shutter blade 101 to gradually open the shutter opening 102. When the appropriate exposure has been obtained (time t.sub.4), using information on the level of the available scene light and the like, the stepper motor 108 is again rotated in a reverse direction. The pin 105 then follows the cam 107 whereby the control lever 103 is operative to allow the shutter blade 101 to close the shutter opening 102. This completes an exposure mode of operation.
With such a conventional single lens reflex camera, the pin 105 of the control member 103 moves into contact with the cam 107 so that the control member 103 can be rotated to open and close the shutter blade 101. This arrangement requires that the stepper motor 108 be rapidly rotated in a reverse direction to drive the shutter blade to a fully open orientation and then rapidly return the same to a closed orientation. The resultant inertia may cause separation of the pin 105 from the cam 107. In other words, if the stepper motor 108 is rapidly rotated in a reverse direction, then the opening and closing of the shutter blade 101 may be delayed. The shutter blade 101 will not follow the stepper motor 108 during high speed rotation. Accordingly, the correct exposure can not be achieved.
To this end, the biasing force of the spring 104 may be increased so as not to separate the pin 105 from the cam 107. However, this results in an increase in the friction between the pin 105 and the cam 107. Also, stepper motor 108 is subject to greater loads. This deteriorates high speed performance of the stepper motor 108 and consumes greater electric current.
In an exposure mode of operation, when the shutter release button is depressed, the stepper motor 108 is rotated in a reverse direction to close the shutter opening 102 by the shutter blade 101. Subsequently, the stepper motor 108 is rotated in a clockwise direction and then in a reverse or counterclockwise direction to open and close the shutter blade 101. In other words, the stepper motor 108 is rotated in a reverse direction to close the shutter blade 101 and immediately thereafter, is rotated in a clockwise direction to open the shutter blade 101. Such a rapid change in the direction of rotation of the stepper motor 108 presents a problem that shutter blade 101 can not immediately respond to the operation of the shutter release button.
In order to overcome the foregoing problems, a shutter for a single lens reflex camera according to the present invention comprises a drive gear rotatable in opposite directions by a motor and having a non-tooth portion, a control gear directly or indirectly engageable with the drive gear, shutter blades driven by the control gear to open and close a shutter opening, and biasing means for constantly biasing the control gear in a direction to close the shutter opening, wherein upon rotation of the drive gear, the control is rotated to face the non-tooth portion of the drive gear whereby the shutter blades are moved under the action of the biasing means to close the shutter opening, and wherein when the control gear is brought into meshing engagement with the drive gear, the shutter blades are rendered operative to open and close the shutter opening.
The control gear is subject to an inertia resulting from rapid opening and closing of the the shutter blades, but positively follows the drive gear since they are directly or indirectly in mesh with one another. The shutter blades can be therefore opened and closed without delay if the direction of rotation of the stepper motor is rapidly reversed.
The biasing force of the existing spring needs not be increased to improve the performance of the shutter blades in response to the stepper motor at high speeds. This avoids application of excessive loads to the stepper motor.
Upon depression of a shutter release button, the stepper motor is rotated in a clockwise direction. This causes the control gear to face the non-tooth portion of the drive gear to thereby drive the shutter blades toward a closed orientation. Thereafter, the stepper motor continues to rotate in the same direction, and then is rotated in an opposite directional sense to open and close the shutter blades. That is, the stepper motor is rotated in a clockwise direction to drive the shutter blades toward a closed orientation once and immediately thereafter, continues to rotate in the same or clockwise direction.